It is common practice in the chemicals handling industries to provide tanks and other vessels with pressure relief vents. One such pressure relief system or vent is the rupture disk. A rupture disk is a relatively thin disk of material with a series of score lines, usually radially positioned, on the generally flat surface thereof. The disk remains intact as long as the pressure differential between the tank interior and the tank exterior remains below a certain amount known as the set pressure. When the pressure differential exceeds the set pressure, the reduced ability of the score lines to withstand the pressure results in the disk rupturing, allowing the pressure differential to be released. Once ruptured, however, the disk does not reclose and must be replaced. This is not an acceptable solution in a situation where the tank or vessel is mobile and may be overturned in an accident or the like, since the irreversible opening of the disk may result in release of a hazardous material such as gasoline.
A second type of relief vent is the re-seatable type of pressure relief vent. Such relief vents have a flat disk seated in an opening in the tank. A spring or other biasing means is used to hold the disk against the seat in a sealing relationship as long as the pressure differential remains below the set pressure. When the pressure differential exceeds the set pressure, the biasing force of the spring or other biasing means is overcome and the disk lifts long enough to relieve the pressure differential below the set pressure, at which time the disk re-seats. When a cargo tank or the like is involved in a motor vehicle or train accident, the tank will often experience a brief surge of transient pressure greatly in excess of the set pressure. If this occurs and the tank or vessel is overturned so that the vent is located below the liquid level in the vessel, the opening of the vent by the pressure surge may result in product spillage.
New U.S. Department of Transportation regulations scheduled to go into effect in August, 1995 will require that the venting system on tanks or vessels be able to withstand a dynamic pressure surge reaching 30 psig above the set pressure and sustained for at least 60 milliseconds without allowing any leakage through the vent (The current regulation, effective in August, 1992, allows leakage under identical circumstances of up to one gallon of liquid). However, the venting system must open at the set pressure when it is exposed to a steady pressure rise in the tank.